The invention relates to a method for adjusting a phase angle of a phase modifier of a transmitting device. The transmitting device comprises a quadrature modulator and a power amplifier which is linearized via a so-called Cartesian feedback loop with a quadrature demodulator.
EP 0 706 259 A1 discloses a transmitting device wherein a basic band input signal is supplied to a quadrature modulator via two differential amplifiers. Said quadrature modulator performs quadrature modulation of the inphase component and the quadrature phase component of the complex input signal. Power amplification takes place in a power amplifier connected downstream the quadrature modulator. To compensate the non-linerarity of this power amplifier a feedback loop is provided, generally designated as a Cartesian which separates the fedback signal into a fedback inphase component and a fedback quadrature phase component. The fedback inphase component is supplied, together with the inphase component of the input signal, to a first differential amplifier, connected upstream the quadrature modulator. Correspondingly, the fedback quadrature phase component is supplied, together with the quadrature phase component of the input signal, to a second differential amplifier. In this way the non-linearities of the power amplifier are compensated via the fedback signal.
In a transmitting device operating according to the Cartesian feedback method it is particularly important that the fedback signal is input inphase. In order to achieve this, the signal of a local oscillator, which is required for the quadrature modulation and quadrature demodulation, is supplied to the quadrature demodulator at a phase angle shifted with regard to the quadrature modulator. The phase shift takes place in a phase modifier, the phase angle of which has to be adjusted. To adjust the phase angle, in EP 0 706 259 A1 a test mode is proposed, in which the feedback loop is interrupted at the output of the quadrature demodulator. A test signal is applied to the input of the quadrature modulator and the output signal of the quadrature demodulator is measured. With a predetermined input signal the phase angle to be set can be calculated from the real part and the imaginary part of the output signal of the quadrature demodulator.
Of disadvantage in the mode of operation proposed in EP 0 706 259 A1, however, is that the feedback loop for determining the phase angle has to be opened each time. This method may be suitable for adjusting the phase angle once on taking into operation, but in the application of a transmitting device operating on the Cartesian feedback principle in aeronautical radio, in particular with digital aeronautical radio operating according to the VDL standard (VHL digital link) in TDMA Simplex mode, there is a necessity to check and possibly re-adjust the phase angle at each transmitting interval (transmitting burst). This cannot be done with the method emerging from EP 0 706 259 A1, owing to the time-consuming separation of the feedback loop and the complicated measurement process.